It was demonstrated previously that the KGF receptor (KGFR) cDNA is identical to that of fibroblast growth factor receptor-2 (FGFR-2) except for 49 consecutive amino acids in the extracellular domain, and that this region of divergence is determined by alternative exons. In contrast to the KGFR, FGFR-2 binds acidic FGF and basic FGF with high affinity, but does not bind KGF. Thus, the KGFR-specific exon (exon K) encodes amino acid residues essential for KGF binding. Recently, synthetic peptides corresponding to part of exon K that antagonize the interaction between KGF and its receptor have been identified. One such peptide antagonist and a monoclonal antibody against KGF that neutralizes mitogenic activity compete for binding to the same site on the growth factor. Thus, the region of the receptor represented by this peptide comprises part or all of the ligand binding site. Furthermore, the monoclonal KGF antibody and heparin (which also blocks KGF mitogenic activity) recognize distinct sites on the KGF molecule, both of which overlap the peptide-defined receptor binding site. The T7 prokaryotic expression system was used to synthesize recombinant KGF (rKGF) in large quantities for detailed structure/function analysis and it was found that heparin binding was maintained despite deletion of up to 29 amino-terminal residues, but lost by deletion of an additional 10 residues. Mitogenic activity of mutants with deletions of up to 10 residues was comparable to that of full-length rKGF. Deletion of 29 residues reduced KGF-induced KGFR tyrosine-kinase activity and DNA synthesis, although high affinity receptor binding was maintained. The characteristics of this partial agonist may be useful in the development of competitive antagonists of KGF action. Purification of substrates of the hepatocyte growth factor receptor and analysis of the mechanism of action of a naturally-occurring HGF antagonist are also currently in progress.